1. Field of the Invention
This invention relates generally to the simulation of a semiconductor fabrication process and, in particular, to the optimization of simulation run-times based on fuzzy-controlled input values.
2. Description of the Related Art
Manufacturers of semiconductor devices are generally placed under a great deal of pressure by their customers for immediate product deliveries. Late product deliveries, or deliveries with less than the ordered number of products, result in lost business to competitors and could lead to financial hardship. One way to avoid production delivery delays is to ensure that the semiconductor fabrication and assembly process is efficiently operated such that all of the manufacturer's production obligations can be satisfied.
Accordingly, manufacturers of semiconductor devices have begun to use computer simulations in an attempt to efficiently manage their semiconductor fabrication and assembly process. Typically, the simulation will model the physical configuration of the fabrication and assembly lines. This model is used to identify potential bottlenecks in the process while also maximizing the equipment and personnel such that "lots" of fabricated and assembled products are efficiently produced. Lots are subsequently used to satisfy product delivery schedules. Since lots may be designated for a particular delivery, they are assigned due dates. Lot due dates, and sizes, as well as resource availability and other production requirements are also used by the simulation to generate dispatch lists. The dispatch lists, which are used by the fabrication and assembly line personnel, include information required to operate their portion of the manufacturing process such that all production obligations are timely satisfied from the produced lots.
To maintain the accuracy of the dispatch lists, the simulation must be periodically rerun to take into account current conditions. Presently, manufacturers will rerun the simulation every few hours regardless of the state of the actual assembly process. A simulation period or "run-time" of every few hours is used so that the assembly line personnel are not constantly working with new dispatch lists and, therefore, are not constantly modifying their portion of the manufacturing process. This is fine when the state of the manufacturing process remains constant. The predefined constant run-time, however, does not bode well when outside factors drastically affect the fabrication and assembly process.
The fabrication and assembly process may be effected by outside forces such as, for example, changes in customer orders, equipment that is broken or not working up to desired specifications and loss of operating personnel. All of these factors, as well as other unforeseen problems, effect the flow of the fabrication and assembly process and need to be accounted for such that new dispatch lists can be created. As stated earlier, manufacturers use a predefined constant run-time, e.g., every few hours, regardless of the state of the actual assembly process. Therefore, if immediately after a simulation has been run, an outside force such as broken equipment alters the flow of the assembly process by producing defective products, for example, it will be at least a few hours before the simulation is rerun. During this time, the assembly process will be producing lots inefficiently. In addition, it may take several simulation reruns to properly modify the fabrication and assembly process such that it returns to an efficient mode of operation. Again, during this time, lots are being produced that are either incorrect in size, or way ahead or behind schedule.
Accordingly, there is a desire and need to optimally adjust simulation run-times based on input parameters generated from outside forces that are impossible to integrate into a simulation model such that longer simulation run-times are used when the flow of the manufacturing process is going well and shorter simulation run-times are used when the flow of the manufacturing process is not going well.